Gene regulation (selective activation and inactivation of genes) plays an important role in the development and progression of cancer, an assemblage of diseases that result from multiple accumulated mutations. The past two decades have witnessed the rapid expansion of our knowledge of cancer genetics, from a handful of oncogenes to the identification of many genes that affect tumorigenesis, tumor growth, progression, metastasis, and tumor cell death. Elucidation of the molecular mechanisms underlying these events provides the opportunity to develop new mechanism-based therapeutics. As a result, the first molecular targeted agent (Trastuzumab) is in clinical use, and many molecular-based agents are in clinical trial.
An embodiment of the present invention is the discovery and characterization of potential chemotherapeutic agents that specifically target tumor hypoxia. The existence of hypoxic regions is a common feature of solid tumors. Unlike normal cells from the same tissue, tumor cells are often chronically hypoxic. The extent of tumor hypoxia correlates with advanced stages and poor prognosis. Rapid growth of tumors outstrips the capability of existing blood vessels to supply oxygen and nutrients, and remove metabolic waste. Hypoxia triggers tumor angiogenesis and the newly formed tumor blood vessels often fail to mature. As a result, certain tumor regions are constantly under hypoxic stress due to sluggish and irregular blood flow. Hypoxic tumor cells are more resistant than normoxic tumor cells to radiation treatment and chemotherapy and these hypoxic cells are considered an important contributor to disease relapse. Currently, the general strategies to overcome tumor hypoxia are: 1) increasing tumor oxygenation by means such as breathing carbogen (95% O2, 5% CO2); 2) developing chemical sensitizers to increase the sensitivity of hypoxic cells to radiation; and 3) developing hypoxic cytotoxins that selectively kill hypoxic cells. These approaches target the direct effects of hypoxia—lack of cellular oxygen. Presently, there is only one bioreductive drug (tirapazamine) in clinical trial that selectively kills hypoxic tumor cells. No hypoxic cytotoxins are currently approved. It is clear that tumor hypoxia is an important unmet therapeutic need for cancer treatment and drug discovery efforts should be directed at this target.
The focal point of this drug discovery effort is to target the important indirect effect of hypoxia—induction of genes that promote the adaptation and survival of tumor cells. As a form of stress, hypoxia activates both survival and cell death programs. In oncogenically transformed cells, hypoxia provides a physiological pressure and selects for the cells with diminished apoptotic potential. Hypoxic tumor cells that have adapted to oxygen and nutrient deprivation are associated with a more aggressive phenotype and poor prognosis. The transcription factor that plays a critical role in hypoxia-induced gene expression is Hypoxia-Inducible Factor-1 (HIF-1), a heterodimer of the bHLH-PAS proteins HIF-1α and HIF-1β/ARNT. HIF-1α protein is degraded rapidly under normoxic conditions and stabilized under hypoxic conditions, while HIF-1β protein is constitutively expressed. Upon hypoxic induction and activation, HIF-1 binds to the hypoxia response element (HRE) present in the promoters of target genes and activates transcription. Survival genes activated by HIF-1 can be classified into three major functional groups—(i) those that increase oxygen delivery through enhancing angiogenesis, erythropoiesis, and vasodilatation; (ii) those that decrease oxygen consumption through inducing numerous genes involved in anaerobic metabolism (glucose transporters and glycolytic enzymes); and (iii) growth factors. In addition to hypoxia, other tumor-specific mechanisms that increase HIF-1 activity include the activation of oncogenes (i.e. ras, src, myc, etc.) and the loss of tumor suppressor genes (i.e. PTEN, VHL). The oxygen regulated subunit HIF-1α protein is overexpressed in common human cancers and their metastases, and is associated with advanced stages in breast cancer. In animal models, deletion of either HIF-1α or HIF-1β blocks hypoxic induction of the genes that are normally induced by hypoxia, and is associated with reduced tumor vascularity and retarded tumor growth. In addition, inhibition of HIF-1 function through blocking the interaction between HIF-1 and the coactivator p300/CBP leads to an attenuation of hypoxia-inducible gene expression, reduction of angiogenesis, and suppression of both breast and colon carcinoma cell-derived tumor growth in vivo. In summary, results from multiple animal models indicate that inhibition of hypoxia-induced gene expression through blocking HIF-1 production/function is associated with significant suppression of tumor growth. Therefore, small molecule specific inhibitors of HIF-1 represent potential chemotherapeutic drugs that will suppress tumor growth, progression, and hypoxia associated treatment resistance by inhibiting hypoxia-induced gene expression.